Higher plants are autotrophic organisms that can synthesize all of their molecular components from inorganic nutrients obtained from the local environment. Nitrogen is a key element in many compounds present in plant cells. It is found in the nucleoside phosphates and amino acids that form the building blocks of nucleic acids and proteins, respectively. Availability of nitrogen for crop plants is an important limiting factor in agricultural production, and the importance of nitrogen is demonstrated by the fact that only oxygen, carbon, and hydrogen are more abundant in higher plant cells. Nitrogen present in the form of ammonia or nitrate is readily absorbed and assimilated by higher plants.
Nitrate is the principal source of nitrogen that is available to higher plants under normal field conditions. Thus, the nitrate assimilation pathway is the major point of entry of inorganic nitrogen into organic compounds (Hewitt et al. (1976) Plant Biochemistry, pp 633-6812, Bonner, and Varner, eds. Academic Press, NY). Although some plants directly utilize ammonia, under certain conditions, nitrate is generally the major form of nitrogen available to plants.
Nitrate uptake by root cells is the first step of the nitrate assimilation pathway in higher plants (Orsel et al. (2002) Plant Physiology 129: 886-896). Plants have developed two different uptake systems to cope with the varying availability of nitrate in cultivated soils. The low-affinity nitrate transport system is used preferentially when external nitrate concentration is high, whereas the high-affinity transport system (HATS) takes place at very low external concentrations.
In higher plants, two gene families have been identified: the NRT1 and NRT2 families involved in the low-affinity transport system and HATs, respectively. The complexity of nitrate/nitrite transport is enhanced by the fine regulation that occurs at the transcriptional level: both low and high-affinity systems have constitutive and inducible components that are clearly distinct. Furthermore, some members of the nitrate transporters require a second gene product, a NAR2-type polypeptide for function (Tong et al. (2005) The Plant Journal 41: 442-450).
The nucleotide sequences of the instant application and the methods of their use can increase the efficiency by which nitrogen can be used.